Side Chains Override Crystallinity in n-Type Organic Mixed Conductors.

In organic semiconductors, crystallinity is commonly associated with enhanced charge transport. For organic mixed ionic-electronic conductors (OMIECs), materials at the core of bioelectronic devices, whether higher crystallinity consistently translates into improved performance remains unresolved. Here, we use thermal annealing to control the crystallinity of three electron-transporting OMIECs bearing either branched or linear ethylene glycol side chains that are used to promote ion transport. Although annealing uniformly enhances crystallinity across all materials, it improves mixed charge transport only in polymers with linear side chains by doubling electronic charge mobility. Specifically, annealed films with branched side chains exhibit reduced mobility and low water uptake, coinciding with a pronounced bipolaron formation, which we uncovered using a combination of in-operando physicochemical characterization methods. Thermal annealing is also used to sterilize these materials for interfacing with living cells, with the benefit of improved sensor performance. These results reveal that crystallinity can hinder mixed conductivity depending on side-chain architecture, independent of the backbone chemistry. By challenging the prevailing assumption that crystallinity is universally beneficial for charge transport, this work establishes design rules for developing OMIECs that combine high performance with compatibility for fabrication and sterilization processes involving high temperatures, paving the way for reliable, scalable bioelectronic devices.